Fourier transform interferometric spectrometers are widely used in the analysis of chemical compounds. By measuring the absorption of radiation by an unknown sample at various wave lengths and comparing the results with known standards, these instruments generate useful information with respect to the chemical makeup of the unknown sample.
Conventional Fourier-transform (FT) spectrometers are based on Michelson interferometer arrangements. In a typical FT spectrometer, light from a light emitting source is collected, passed through an interferometer and sample to be analyzed, and is brought to focus on a detector (Saptari, Vidi, Fourier-Transform Spectroscopy Instrumentation Engineering. SPIE Press, Bellingham Wash., Vol. No.: TT61 (2004), herein incorporated by reference in its entirety). The interferometer system, in combination with the sample, modulates the intensity of the light that strikes the detector, and thereby forms a time variant intensity signal. A detector, analog-to-digital converter, and processor then receive, convert, and analyze the signal.
In an FT spectrometer utilizing an interferometer (SEE FIG. 1), input light is divided into two beams by a beam splitter. One beam is reflected off a fixed mirror and one off a moving mirror. While the path length of the beam striking the fixed mirror is substantially constant, movement of the moving mirror alters the path length of the other beam, thereby changing the distance the beam travels in comparison to the reference beam (e.g., the beam reflected off the fixed mirror. This changing path length introduces a time delay between the beams upon reaching the detector. The time-offset beams interfere or reinforce each other, allowing the temporal coherence of the light to be measured at different time delay settings, effectively converting the time domain into a spatial coordinate. Measurements of the signal at many discrete positions of the moving mirror are used to construct a spectrum using a FT of the temporal coherence of the light. The power spectrum of the FT of the interferogram corresponds to the spectral distribution of the input light. The moving mirror allows a time-domain interferogram to be generated which, when analyzed, allows high resolution frequency-domain spectra to be produced. A Fourier transform is performed on the data to produce a spectrum which shows spectral-energy versus frequency.
It is critical in the design of these instruments that the surface of the moving mirror be very accurately held in an orthogonal position, both to the fixed mirror and to the direction of the motion of the moving mirror. Mirror positional accuracy is of importance because deviations in the mirror alignment produce small errors in the time-domain interferogram which may translate into large errors in the frequency-domain spectrum. In a typical interferometer, mirror deviations larger than one wave length of the analytical radiation are considered significant and can degrade the quality of the entire instrument. Contemporary high- and moderate-performance Fourier-transform spectrometry instruments may utilize stabilization assemblies, typically flexure assemblies, to support the moving mirror in the interest of minimizing mechanical hysteresis and other non-linear effects.
Despite all measures taken to reduce undesired alteration of the moving mirror's position, some degree of unwanted or unintended deviation may occur in even high quality systems. The motion of the mirror in response to applied force is described by a second-order differential equation whose coefficients are determined by the mass of the moving mirror, the effective spring constant of the flexures, the frictional loss introduced by suspension, and viscous loss introduced by motion through the atmosphere. Forces directed at the moving mirror may be a result of a command sent to the mirror actuator (e.g., a voice-coil actuator) as part of the normal function of the FTS instrument, or may be undesired disturbances, caused, for example, by incidental motion of the entire instrument. The frictional losses present in the flexure suspension are very low, a condition which leads to a lightly-damped system response to applied force. Such a system is significantly more difficult to control than one with a greater level of inherent damping. The lightly-damped system is also more susceptible to externally-produced disturbance forces than one with greater damping.